Driven by emission legislation and market force, specific power and peak cylinder pressure of light and heavy duty internal combustion engines have increased over the past few decades. Factors that have contributed to these increases include improvements in crankshaft design, improvements in bearing design, advances in bearing materials, and relatively more efficient manufacturing techniques.
In part, research focuses on reducing friction in the crankshaft bearing through thinner bearings in conjunction with reduced lubricant viscosity, wherein lubricant viscosity may be reduced by modifying the oil blends and/or by operating at relatively higher temperatures. In addition to plain bearings, roller bearings have been investigated for application in crankshafts, particularly for main journal support. Typically, such research is accomplished through a combination of simulation and experimental techniques to take proposed designs and material formulations from concept to testing on fired engines.
In one experimental technique, a bearing may be installed in a testing rig and a lubricant is added. The bearing rotates while a low frequency, sinusoidal load is applied and the bearing is generally subjected to static misalignment. Bearing wear and lubricant characteristics, such as lubricant viscosity and degradation, may be measured during the course of testing. However, these devices generally do not simulate the operating conditions of the bearings and lubricants in combustion engines and the correlation to engine wear or fatigue life is relatively poor. Engine testing on the other hand, may be relatively complex. The tests may be relatively costly, the ability to instrument may be difficult, and the ability to separate control and variables of interest may be difficult.
In combustion engines, the load magnitude on a bearing may be relatively large, such as in the range of 20 kN to 300 kN and the loads applied to the bearings change relatively rapidly, such as in the range of (+/−) 20 kN/ms to (+/−) 200 kN/ms. Often devices that are capable of generating rapidly varying loads, like solenoids, piezoelectric actuators, voice coils, etc., are not generally capable of producing the forces experienced in a combustion engine. Systems that may be powerful enough to provide loads close to those exhibited in combustion engines may lack in other areas. For example, mechanical systems may not be able to cover a wide enough load range and speeds. Hydraulic systems may not exhibit a high enough rate of load change due to relatively large flow rates of the hydraulic fluid as well as due to the compliance of the oil and testing apparatus.
Accordingly, room for improvement remains in the development of a bearing test rig that may be capable of replicating the load history experienced by cranktrain bearings. Such rigs may utilize actual components when possible, supply a lubricant through the testing shaft, and enhance pressure to replicate centrifugal forces. Further, such testing may better quantify start-up and shut-down wear, scuff onset and scuff onset conditions, and bearing fatigue life under various conditions.